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Wednesday, 3rd September 2014

What You Need to Know about Thunderstorm Warning

"The Observatory issued thunderstorm warning at one o'clock this afternoon......". During summer, you may often come across such warning messages on radio, the Observatory's website or mobile application "MyObservatory". However, on some occasions, you were out there in the sun without any sign of thunder or raindrop. You might ask doubtfully, "Where have all the thunderstorms gone?"

As a matter of fact, it is common to see sunshine and shower concurrently in Hong Kong, especially during the heat of the summer. In inland areas that heat up more quickly in the day, showers and thunderstorms are easily triggered due to high temperature. Sometimes, this kind of thunderstorms is limited in size, and thus weather at a distance may remain fine without showers or thunderstorms (Figure 1). When the Observatory issues the thunderstorm warning, it aims at alerting the public of the threat posed by thunderstorms within the territory. Where possible, the Observatory provides additional information on the specific areas affected by thunderstorms in the warning bulletin and the weather report (Figure 2). Under the situation as illustrated in Figure 1, people in the urban areas are not aware of the thunderstorms that are witnessed by people in Sai Kung. On some occasions, thunderstorms are affecting only part of the territory but they are forecast to evolve quickly so that there is a chance of them spreading across the territory or affecting more places in Hong Kong. For safety reason, the Observatory will not specify the areas affected by thunderstorms in the warning. This eliminates the need to update the warning frequently, which may cause confusion to the public.

Figure 1

Figure 1      Radar images showing the rapid growth and decay of thunderstorms. Lightning locations
are denoted by white squares.

Figure 2

Figure 2      Members of the public can check the thunderstorm warning and the affected
areas using "MyObservatory".

How should the public respond when thunderstorm warning is in force? If you are located within the affected area as mentioned in the warning, please take appropriate precautions such as staying indoors or stop engaging in water sports. For details, please visit the Observatory's webpage on thunderstorm warning: Whenever you want to know if there are any thunderstorms close by that may affect your outdoor activities, please take a look at the lightning location map in the Observatory's website ( or use the mobile application "MyObservatory" to check the location of lightning associated with thunderstorms. By keeping an eye on the news of thunderstorms and making good use of the Observatory's online information, members of the public can better plan and engage in outdoor activities.

The growth and decay of thunderstorms can be very rapid and sporadic. It is still a challenge to forecast accurately their variation several hours ahead owing to the limitation of the existing science and technology. Therefore, the validity period of Observatory's thunderstorm warning will not be too long to minimize false alarm and reduce disruption to the public. The Observatory will keep close watch on the evolution of thunderstorms and extend the warning period if needed, until the thunderstorms completely dissipate to ensure public safety.

YC Kong


Friday, 20th June 2014

Anticipated Return of El Niño*

Four years after the previous El Niño event, significant warming signs re-emerged over the central and eastern equatorial Pacific in the past couple of months. The sea surface temperature in May 2014 has already exceeded the normal range over those regions, suggesting that another El Niño is brewing (by definition, an El Niño occurrence is confirmed on the basis of anomalously warm central and eastern equatorial Pacific persisting for five to six months). As the warming trend is expected to continue in the next few months as predicted by many climate models around the world, it is therefore likely that an El Niño event will become established later this year.

Incredibly, an anomaly in the ocean so far away can alter the atmospheric circulation worldwide through what is known as "teleconnection" mechanism. This in turn will affect the weather and seasonal climate in many parts of the world, including Hong Kong to a certain extent.

Will El Niño bring more rainfall?

People tend to associate El Niño with heavy summer (June-August) rainfall because a strong El Niño in 1997 coincided with the wettest summer (2358.6 mm) and the wettest year (3343.0 mm) on record in Hong Kong. However, the summer of 1983, another intense El Niño year, had below-normal seasonal rainfall of just over 900 mm. Looking at the data over a long period of time, the general relationship between El Niño and Hong Kong's summer rainfall remains inconclusive. An analysis of El Niño summer rainfall records during 1950-2013 shows that the percentage occurrence of a drier summer is even higher than that of a wetter summer (Figure 1), although the difference may not be considered statistically significant taking into account the limited records available.

On the other hand, El Niño's impact on winter (December-February) and spring (March-May) rainfall is more pronounced. Figure 2 shows the distribution of Hong Kong's winter rainfall under El Niño and normal conditions (i.e. neither El Niño nor La Niña, the opposite phase of El Niño) during 1950-2013. It can be seen that the odds of a wetter (drier) El Niño winter will noticeably increase (decrease) compared to a normal winter.

Will El Niño affect tropical cyclones?

Since 1961, there has not been one single tropical cyclone coming within 500 km of Hong Kong in April and May when El Niño is in place (see Figure 3), whereas in a La Niña or normal year, the tropical cyclone season in Hong Kong can begin as early as April, e.g. 1978. That is because tropical cyclones in El Niño years tend to form further east near the central North Pacific and hence are less likely to journey as far as the South China Sea in the early season. As the season progresses, other factors come into play and El Niño's impact on tropical cyclone behaviour then becomes less well-defined.

For more information on El Niño and La Niña, please visit the Observatory webpage:

S M Lee

* The warming of surface waters over the central and eastern equatorial Pacific Ocean usually peaks around Christmas, hence the name "El Niño" (Spanish for "the little boy" or "the Christ Child") coined for the phenomenon.

Figure 1

Figure 1      Distribution of Hong Kong's summer rainfall under El Niño and normal conditions during 1950-2013.
Normal condition refers to the situation of neither El Niño nor La Niña in place
during the whole season.

Figure 2

Figure 2      Distribution of Hong Kong's winter rainfall under El Niño and normal conditions during 1950-2013.
Normal condition refers to the situation of neither El Niño nor La Niña in place
during the whole season.

Figure 3

Figure 3      Average monthly number of tropical cyclones coming within 500 km of Hong Kong
under El Niño and normal conditions during 1961-2013. Normal condition refers
to the situation of neither El Niño nor La Niña in place during the month.


Friday, 13th June 2014

Take Precautions against the Heat

You may have noticed that the Observatory recently issued Special Weather Tips on hot days to remind members of the public to take precautions against the heat. This is a new service launched by the Observatory this summer - the "Hot Weather Special Advisory". It aims to raise public awareness of taking appropriate precautions when the weather is rather hot but not to the extent that warrants the issuance of Very Hot Weather Warning. In fact, according to press reports from 2005 to 2013, the average number of people suffering from heat-related illnesses was about 75 per year, out of which around 4 people died per year. Indeed, it is crucial to be alert of the impact of hot weather to health. Details of the Observatory's new service and the precautions against hot weather can be found via the following link:

When issuing the "Hot Weather Special Advisory" or the Very Hot Weather Warning, the Observatory considers basically the same factors but with different degrees. It is not a must for a "Hot Weather Special Advisory" to be followed by a Very Hot Weather Warning; it depends on whether the weather will become hotter. The concerned factors include the atmospheric environment, the weather conditions at different places in Hong Kong, and the trial Hong Kong Heat Index. The index represents the combined effect of temperature, humidity, wind speed and solar radiation; and is calculated from the natural wet bulb temperature, the globe temperature and the dry bulb temperature measured at King's Park Meteorological Station. Real-time Hong Kong Heat Index can be found in the Observatory's webpage:

Figure 1

Figure 1      Observatory's website displaying real-time Hong Kong Heat Index.

Up to this point, you may recall that the Observatory once made reference to the Wet Bulb Globe Temperature (WBGT) when providing services for the 2008 Olympic Equestrian Events. The calculations of WBGT and the Hong Kong Heat Index are indeed based on similar parameters. The former was developed based on the climate of western countries, while the latter was developed by taking into account the relationship between local hospitalization rate and meteorological observations. This new index was a concerted effort of the Observatory and local university, in consultation with different stakeholders. It was more applicable to the environment of Hong Kong, which is characterized by the generally higher humidity.

Since the measurement of hot weather at King's Park is closer to the average value over different regions in Hong Kong, and that the Hong Kong Heat Index was developed by using data from King's Park and the territory-wide hospitalization counts, the index could represent the overall condition in Hong Kong. Generally speaking, when the forecast index is about to reach 30, the Observatory will consider issuing the "Hot Weather Special Advisory" after taking into account a range of other factors. If the index is forecast to be even higher, the issuance of Very Hot Weather Warning will be considered. The Hong Kong Heat Index is currently used on a trial basis. Since there is only limited research data, further categorization of the index cannot be made at this moment. The Observatory will continue to collect more data for evaluation and research with a view to further enhancing the hot weather service in the future.

LS Lee and KL Lee


Last revision date: <03 Sep 2014>